Amphibious Caterpillars

In rocky, high-mountain streams on the islands of Hawaii, Daniel Rubinoff and Patrick Schmitz found tiny caterpillars grazing on algae and lichens underwater, several feet from the bank or surface—a distance that would take the slow-moving animals several days to cover. “A caterpillar can’t hold its breath that long,” says Rubinoff. He and Schmitz, both entomologists at the University of Hawaii, took dozens of the larvae back to their laboratory. To their surprise, the creatures were perfectly fine living underwater in an aerated aquarium for a month or more—but they were also just as healthy residing in a bone-dry tank for the same extended period. These caterpillars are thus the first insects discovered to be truly amphibious, as the group reported in the March 22 online edition of the Proceedings of the National Academy of Sciences of the U.S.A.

The caterpillars belong to a moth genus called Hyposmocoma that is found only in Hawaii. The genus is quite diverse and expansive, encompassing about a third of all butterfly and moth species on the islands (including the only caterpillar species in the world known to predate on snails, which Rubinoff discovered several years ago), but it has received relatively little study. The caterpillars in this genus spin silk cases around themselves, into which they incorporate local materials such as lichens, algae and diatoms, possibly to aid their camouflage and protection. So far there are three groups of yet-to-be-named species of amphibious Hyposmocoma, each of which builds its case in a different shape—bugle, burrito or cone. Rubinoff and Schmitz did DNA studies on adult moths and found that each group seems to have evolved its aquatic abilities independently, not from a common ancestor, and each species is endemic to a specific volcano habitat on each of the islands.

The caterpillars live in areas with volatile hydrodynamics: A rock that is three feet out of the water today could be submerged tomorrow, and there are frequent flash floods. Often finding themselves either deluged or marooned, the larvae make good use of their amphibious flexibility. In the water, the animals use silk threads to anchor themselves against currents, and they often gather together in small holes on the downstream sides of the volcanic rocks. Their cases do not store air bubbles that could account for long underwater activity. On dry land the insects breathe using spiracles, small holes along their bodies, but underwater, they appear to absorb oxygen by direct diffusion through their skin. Therefore, they can only survive in highly aerated, fast-flowing water. “Part of the catch for these things in straddling the aquatic- terrestrial environment is that they need specific conditions to be happy underwater,” says Rubinoff.

Although the caterpillars currently live in mountainous areas, Rubinoff suspects these are a remnant of their original range, as the animals can still be found in a few lowland streams that have been preserved intact. It’s likely that over the last thousand or so years of human occupation of the islands, streams diverted for agriculture became too stagnant for the caterpillars—or too dry for their algal food—to survive. Their specific habitat needs may be particularly important to preserve, says Rubinoff, in order to get a better picture of the process of evolution on islands.

“Islands generate diversity that’s not found on any mainland area, and that makes them even more important for understanding how evolution functions. Hyposmocoma represent one of the last relatively complete pictures of how complex the evolution of a group in those islands could be,” he says. “We’ve got these species that are not closely related to each other, which are living close by or adjacently, doing things that we see nowhere else on the planet, and it seems like that’s got to be worth something.”